1. Field of the Invention
The present invention relates to a color developer and a recording unit having a layer of the color developer coated on a support.
2. Description of the Prior Art
There have been known recording sheets utilizing a color reaction of a colorless organic compound (hereinafter referred to as a "color former") and an adsorbent material capable of forming a distinct color when contacted with the color former.
As the recording sheets using this phenomenon, a pressure-sensitive recording sheet (U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,548,366, 2,712,507, 2,730,456, 2,730,457, 3,418,250, etc.) and a heat-sensitive recording sheet have been well known. Further, a printing method is known where an ink, containing the color former, is applied to the color developer sheet through a medium such as a stencil.
In all cases, the above phenomenon of the color reaction between the color former and the color developer are utilized under pressure with a pen or stylus, under heat, etc.
The color former may be dissolved in a solvent such as chlorinated diphenyl, chlorinated paraffin or other organic solvent. The solution may be dispersed in a binder and/or may be microencapsulated, and then coated onto a support such as paper, plastic film, resin-coated paper, etc.
In case of heat-sensitive recording sheets, the color former may be coated onto the support together with a thermofusible material such as acetoanilide which is melted under heating to dissolve the color former.
On the other hand, the color developer is dissolved or dispersed in water or in an organic solvent together with a binder such as a styrene-butadiene rubber latex and then coated on or impregnated into a base support before or directly before recording.
The color former and color developer may be coated onto the same surface or opposite surfaces of a support, or different supports.
Usually, after the color developer is coated onto the support, a desensitizer is locally coated thereon in order that unnecessary color formation be prevented in certain portions of the surface (U.S. Pat. No. 2,777,780). Another embodiment for the prevention of color formation is to locally coat the color developer on the support or the coated layer of color former without using the desensitizer.
As the above described color developer, there are clays such as acid clay, active clay, attapulgite, zeolite, bentonite, etc.; phenol resins (U.S. Pat. Nos. 3,516,845 and 3,540,911); and organic compounds such as succinic acid, tannic acid, gallic acid or phenol compounds (U.S. Pat. No. 3,244,548) are known. Such organic compounds are not practically employed for the reason that the color developing ability of the color developer sheet which is a support having coated thereon the color developer is very low and the color developer on the sheet is low in light resistance although the compound can be very easily handled in manufacturing the color developer sheet.
Further, in cases where the above color developer is locally coated onto a support to form a color developer sheet, only if a large amount thereof (7-10 g per 1 m.sup.2 of the support) is coated, the color developer sheet can form sufficient color when contacted with the color former. Therefore, since locally coated portions of the color developer sheet are thicker than other portions, the color former coated on another support is locally pressurized to form unpreferable color (hereinafter referred to "fog") when contacted with the color developer sheet. As a result, the local coating method is limited.